Art of forming dielectric films.



UNITED srna ns 'rnn'r FEICE RALPH n. rannsnon, on NEW roan, N. Y,

ART .OF FORMING DIELECTRIC FILMS.

No Drawing.

Specification of Letters Patent. Patented Dec, 26 1911,. Applicationfiled January 5, 1910. Serial F0. 536,442.

To all whom it may concern:

Be it known that I, RALPH D. MERSHON, a citizen of the United States,residing at New York, in the county and State of New York, have inventedcertain new and useful Improvements in the Art of, Forming DielectricFilms, of which the following is a full, clear, and exact description.

As is well known, the action of electrolytic condensers, rectifiers, andsimilar devices depends upon the film which may be. formed upon thesurface of aluminum, tantalum,

magnesium and other metals when immersed in certain electrolytes andsubjected to the electric current; which film possesses the singularproperty of allowing current to flow from the electrolyte to theelectrode with but I little opposition, but of strongly opposingcurrent-tending to flow from the electrode to the electrolyte. I Tocurrent tending to flow in the direction last indicated the film,although excessively thin, ossesses high specific resistance and highielectric strength, being able to Withstand very considerable voltageswithout rupture, and with very inconsiderable leakage current. It hasbeen found, however, that in order that the film shall evidence to anefiicient degree I the qualities of high'specific resistance and highdielectric strength its temperature must not be much, if any, above acertain value; and that as the temperature approaches such value (about40 C. in the case of the electrodes andelectrolytes most commonly used)the film begins to lose these good qualities, the impairment of thebehavlor of the film above this critical tem rature being'very marked.Inall the devices depending upon the existence of such avfilm for theiraction there is a certain amount of 10s, the loss ing ingeneralgreaterwith alternating than with direct current. The resulting heatmust belgot rid of by means of radiation or convection to thesurrounding air, or by means of coils in which water, oil or othermedium is circulated, or by means of an air-blast," etc. But suchtransference of heat means that'the heat-receiving medium 'must be at alower temperahlre than the electrolyte, and inasmuch as the temperatureof the air will often be near to and sometimes above that at which, aspreviously stated, the behavior of the film rapidly deteriorates inefiectiveness, air cooling will not in general be satisfactory for thepurpose, as a small temperature difierential would require a very largearea of cooling surface, and even this large surface wouldtemperature isual to or higher than the critlcal value. ooling by means of water oroil circulation will at times be subject to like objections, though notto so great a degree. This sensitiveness of the filmto heat has been oneof the most serious ditliculties encountered in the use of electrolyticapparatusof the kind mentioned, and to avoid or obviate the same haslong been a desideratum in this art. considerable experimentation andstudy in this direction, and in the course of the same I have discoveredthat an important relation appears to exist between the sensitiveness ofthe film to heat and the temperature to which it is subjected in thecourse of its format-ion or preparation; namely, that, in general, thehigher the temperature during formation or preparation of the film thehigher will be the temperature which the film can afterward withstandwithout seri- I have therefore been led to i ous, if any, impairment ofits essential f properties. It'is upon this discovery that the inventionwhich forms the subject of my present application for Letters Patent isbased.

of aluminum (or other suitable metal) and "both immersed :in a suitableelectrolyte.

One of these electrodes or plates is attached to one terminal of the.alternati current circuit and the other plate is at d to the Brieflystated, the invention consists in .forming or preparing the. films at atemperature above that at which they are after-.

other terminal. Where direct current is to be used to form the films allthe electrodes which are to be filmed are connected tothe positive poleof the direct current source and to theother 1e is connected a cathodeelectrode, pro erably of non-filming. mate rial, say of carbon, iron,lead, or other direct current this'limit approximates zero,

. t, able material. In eithercase an E. M. F. of

or at any rate has a very low value. (With alternating current the limitis higher, partly because of the hi her losses with alternating currentbut chiefiy because of the charging or condenser-current component. Ingeneral it is better, when using alternating current, to depend upon theindications of a wattmeter rather than an ammeter, as wattmeter readingswill not be masked by the large charging component.) If, after thecurrent has ceased" to fall rapidly, the E. M. F. be increased thecurrent flowing will increase ,coincidently, again falling toward alimitmg value. By increasing the voltage in this manner, that is, byrelatively small steps, or by leaving at any step a constant voltageuntil the current has fallen to an approximately constant value in thecase of alternatlng current, or to a very low value in' the case Ofdirect current, a film of maximum resistance and dielectric strength maybe built up, capable of withstanding very considera le voltages, asbefore stated. There is, however, a certain maximum voltage, depending,as is well known, upon the kind of metal used in the filming electrodesand the chemical used in the electrolyte, at which the film breaks downand beyond which the forming process cannot be carried. It is of courseclear that .the forming voltage should not exceed this maximum value.

In practlcmg my present invention I pre-' fer to proceed by thestep-by-step method described above, the maximum forming voltageemployed being preferably higher than the maxlmum voltage to which theplates or electrodes will be subjected in the condenser orotherapparatus. The temperature of the plates is then raised, as by heatingthe electrolyte 1n any convenient manner, above the maximum temperatureto which they will be subjected in the electrolytic apparatus for whichthey are designed. By preference the forming temperature is from fiftyto a hundred per cent. above the maximum temperature of use. Thus, if acondenser, for example, is to operate at a temperature as high 'as 60C., the forming temperature would be preferably from 90 to 120 (1, thusalfording ample margin between the operating temperature and the highesttemperature at' which the film is operative. The maximum formingvoltage, too, may be considerably higher than the operating voltage, asbefore stated, but of course it should not reach the value at which thefilm breaks down; The time required to fully form the film dependslargely upon the metal and the electrolyte, but it may be said that ingeneral it is safe to stop when with the highest temperature and voltageemployed in the formation the current from the filming electrode orelectrodes to the electrolyte ceases to fallrapidly; or at any rate whenthe leakage current becomes approximately constant.

In practice it is found that films formed as described are practicallyindependent of temperature conditions below a certain limit, of slightlyless value than but determined by the value .of the forming temperature,and hence may be effective .at temperatures far in excess of anyheretofore permissible. At the same time the films are more effective atlower temperatures and are considerably less sensitive to otherdeleterious agencies, than are films produced by any prior method withwhich I am familiar.

The temperature treatment may be effected by raising the temperaturegradually, or by raising it step-by-step; and it may be 'done while thefilm is being'developed, or

afterward.- In the latter case I prefer to keep current on theelectrodes, with a voltage approximating, at anyrate, the highestvoltage used in developing the films. There are also some advantages inemploying direct current in developing the films (and also in thetemperature treatment step of the formation process), not onlyas regardsthe qualities of the film itself but also as regards ease andconvenience of manipulation and of the measurements indicative of theprogress of the process. For this reason in the following more detaileddescription of the preferred mode of performing. the temperaturetreatment step, and the phenomena attending the same, direct currentwill be mainly dealt with,. although what is said applies qualitativelyto alternating current as well, and it should be borne in mind thatalternating current can be used if desired. It is also assumed that thefilming electrodes are of aluminum, but any other metal suit able forthe purpose may be used.

1. 5 When the film, developed at ordinary temperature, is subjectedtothe temperature treatment step of the formation process, the, i I

current flowing will increase and at or,

above a certain temperature (usually about 40 (1, as before stated) theincrease will be '1 quite marked and will become more andmore marked asthe temperature rises, the' rate of current increase being more rapidthan the rate of temperature increase. But

on holding the electrolyte at one ofthese higher temperatures thecurrent eventually decreases, at first rapidly and then lessand lessrapidly, in a manner similar to that described in the case of increasevoltage. That is, when the temperature is raised the current willincrease, but if the temperature be then held stationary the currentwill fall, rapidly when the decrease first begins and then less and lessrapidly, approachingfor-each temperature a limiting value. If in thecourse of the temperature treatment the temperature be held at thehighest value considered necessary or advisable under the circumstancesa conditlonof aflfairs will be reached to which the film is beingsubjected. The

in which the current is falling very slowly temperature treatment maynow be discontinued, and it will then be found that the film will beefliciently operative in a condenser, rectifier, or'other apparatus, atany temperature up to the limit before mentioned. While the temperaturetreatment is effective when performed with the electrolytes heretoforeused it is more effective if the electrolyte employed in the treatmentbe acidulated, as by the addition of a small quantity of a suitable acidor acids for example, an electrolyte consisting of a solution of boraxcan be acidulated with boric acid. In general, if A, B, C, D, .etc.,represent salts which can be used to form the films, a solution of anyone or more of these salts maybe acidulated by any one or more of theacids of salts A, B, G, D, etc. Thus with a borax electrolyte phosphoricacid may be used instead of or along with boric acid, and with aphosphate electrolyte boric acid may be employed instead of or withphosphoric acid, etc.

In using direct current in the forming process the cathode may be eitherof filming or non-filming material, and the use of the latter is assumedin the foregoing description. \Vith a cathode of filming material thecathode will be attacked little if any if the electrolyte used isa'cidulated but will be more strongly attacked if the electrolyte beunacidulated. In the latter case the per formance of the condenserelectrodes so filmed will be somewhat better if they are operated in theidentical electrolyte or in any acidulated electrolyte. It thereforeappears that when the films are formed in an unacidulated electrolyte,with a cathode of filming material, the temperature treatment effects achange in the composition of the electrolyte as well as in the nature ofthe films themselves. For example, if with direct current and anunacidulated electrolyte the cathode, instead of being of non-filmingmaterial, be made of metal capable of forming a film in the electrolyteused, say the metal comprising the electrodes which are to be filmed,the cathode is attacked and there is thrown down in the electrolyte aquantity of material more or less lnsoluble.

The bulk of this material in the case of aluminum electrodes consists ofaluminum hydrate, and some iron hydrate is found mixed with it,presumably from impurities in the aluminum. There also appear to besmall quantities of a compound, possibly an aluminate, formed betweenthe aluminum and the baseof the electrolyte, and also small quantitiesof the acid (free) of the electrolyte. Just what the change is in theelectrolyte which causes it to. be effective in the same way as thoughit had been acidulated is not clear, unless it be due to the free acidor the aluminate just mentioned, or both. But it appears to be due tosomething in the precipitate going to the bottom of the electrolyte whencold, for although- I plates having films formed in.the way mentioned,that is, with direct current and a cathode of filming material in anunacidulated electrolyte, are not so effective in a fresh solution, likethat in which the filmformation was begun they are just as ,effective insuch fresh solution if there be added to it the precipitate in question;nor are they so effective in the solution obtained by decanting off thetop portion of the electrolyte in which the precipitate has been formedand has settled to the bottom.

I claim:

' 1. The improvement in the art of forming dielectric films onelectrodes for use inv electrolytic apparatus of the kind described,which consists in forming the films at a temperature in excess of thatto which they are to be subjected in the said electrolytic apparatus.

2. The improvement in the art of-forming dielectric films on electrodesfor use in electrolytic apparatus of the kind described, which consistsin immersing the filmed electrodes in a suitable electrolyte andsubjecting the same to heat at a temperature in ex cess of that to whichthey are to be subjected in said electrolytic apparatus.

3. The improvement in the art of forming dielectric films on electrodesfor use in electrolytic apparatus of the kind described, which consistsin subjecting the films to a temperature from 50 to 100 per cent. higherthan the temperature at which the said electrolytic apparatus is tooperate.

4. The improvement in the art of forming dielectric films on electrodesfor use in perature and voltage in excess of the temperature and voltageat which the said electrolytic apparatus 1s to operate.

5. The improvement in the art of forming dielectric films on electrodesfor use in electrolytic apparatus of the kind described, which consistsin immersing the electrodes in a suitable electrolyte, impressingcurrent on the electrodes, raising the temperature to a point higherthan that at which the said electrolytic apparatus is' to operate, andmaintaining such temperature until the leakage current has fallen to asuitable value.

6. The improvement in the art of forming dielectric films on electrodesfor use in electrolytic apparatus of the kind de scribed, which consistsin immersing the A ing current on the electrodes, raising the voltage ofthe current by successive steps to a point in excess of that at whichthe said electrolytic apparatus is to operate, and raising thetemperature to a point in excess of that to which the said apparatus isto be in electrolytic apparatus of the kind described, which consists inimmersing the electrode in a suitable electrolyte at or below thetemperature to which the said electrolytic. apparatus is to besubjected, im-

presslng current on the electrodes, and gradually raising thetemperature to a point in excess of that to which the electrodes are tobe used in said apparatus.

9. The improvement in the art of forming dielectric films on electrodesfor use in electrolytic apparatus of the kind described, which consistsinimmersing the electrodes in a suitable electrolyte, impressing currenton the electrodes and developing films thereon, and raising thetemperature toa point in excess of that at which the electrodes are tobe used in said apparatus.

10. The improvement in the art of forming dielectric films on electrodesfor use in electrolytic apparatus of the kind described, which consistsin forming the films with direct current and at a temperature in excessof that to which they are to be subjected in the said electrolyticapparatus.

I 11. The improvement in the art of forming dielectric films onelectrodes for use in electrolytic apparatus of the kind described,which consists in forming the films at a temperature and with a directcurrent voltage in excess of. the temperature and voltage at which thesaid electrolytic apparatus is operated.

12. The improvement in the art of forming dielectric films on.electrodes for use in electrolytic apparatus of the kind described,which consists in forming the films in an acid electrolyte at atemperature in excess of that to which they are to be subjected in thesaid electrolyt1c apparatus.

13. The improvement in the art of forming dielectric films on electrodesfor use in electrolytic apparatus of the kind described, which consists-in forming the films in an acid electrolyte with direct current and at-atemperature in excess of that to which they are to be subjected in thesaid electrolytic apparatus.

14. The improvement in the art of forming dielectric films on electrodesfor use inelectrolytic apparatus of the kind described, which consistsin immersing the electrodes in a suitable electrolyte, impressing directcurrent upon the electrodes and developing films thereon, and raisingthe temperature to a point 1n excess of that at which the electrodes areto be used in said apparatus. a

. 15. The improvement in the art of forming dielectric films onelectrodes for use in electro -"iic apparatus of the kind described,which consists in immersing the electrodes in a suitable electrolyte,subjecting the electrodes to increasing direct current voltage todevelop films on the electrodes, and raising the temperture to'a pointin excess of that at which the electrodes are to be used in saidapparatus.

16. The improvement in the art of forming dielectric films on electrodesfor use in electrolytic apparatus of the kind described,

which consists in immersing the filmed elecin the presence of twosubscribing Witnesses.

RALPH D. MERSI-ION.

